Typical amplifier architectures use a high gain amplifier with a feedback network so that the overall loop gain is large in the band of frequencies of interest. This high loop gain may lead to significant stability problems at higher frequencies. Otherwise, the amplifier may begin to oscillate as a result of parasitic phase delays, for example.
The high loop gain is reduced at higher frequencies through appropriate roll off circuitry. Reducing the loop gain over too small a range of frequencies may also introduce sufficient phase shift to destabilize the amplifier. The designer must typically compromise between gain and bandwidth in order to have a functional amplifier at the cost of signal distortion.
Although undesirable, the high loop gain requirement is a result of a highly nonlinear open loop transfer characteristic for the amplifier. This nonlinear open loop gain may be attributed to architectural choices made to improve open loop gain or amplifier power efficiency (i.e., rail-to-rail output capability).
Although the amplifier may be designed as a class A amplifier to improve nonlinearities, class A devices consume large amounts of power from the power supplies. Class AB amplifiers provide better power efficiency but are susceptible to crossover distortion. Moreover, class AB amplifiers still suffer the disadvantages of a large nonlinear open loop transfer characteristic.